PROJECT 4 (ESI) examines the mechanisms by which complement activated platelets drive endothelial activation and monocyte/macrophage proinflammatory differentiation and recruitment, promoting vascular remodeling in pulmonary hypertension (PH). Accumulating data from both human and animal studies by our laboratory and others demonstrate a role for platelets in the initiation and progression of PH, however what drives platelet activation and how platelets mediate vascular remodeling in PH has not been addressed. Project 4 utilizes several novel and innovative in vivo and in vitro approaches employing genetic and pharmacologic methods to examine the role of complement mediated platelet activation in two murine models of PH to test the overall hypothesis that complement-mediated activation of platelets induces endothelial activation and drives the recruitment and proinflammatory activation of monocytes to the pulmonary vasculature promoting pulmonary vascular remodeling and pulmonary hypertension. Complement-mediated platelet activation may occur in inflammatory conditions via complement anaphylatoxin activation of platelets. Whether anaphylatoxins activate platelets and drive platelet activation in PH is unknown and is the focus of aim 1. Endothelial dysfunction is central to the pathogenesis of PH, and endothelial cells from patients and animals with PH demonstrate increased expression of proteins which promote platelet-endothelial adhesion. While the mechanisms for platelet recruitment and adherence to the endothelium is well described in hemostasis and thrombosis, it is poorly understood in inflammatory conditions and has not been evaluated in PH. The goal of aim 2 is to determine the mechanisms of complement mediated platelet activation to platelet endothelial adhesion and how platelet adhesion to the endothelium promotes endothelial activation in experimental PH. Monocyte and macrophage recruitment and accumulation within the perivascular/adventitial space is a consistent feature of pulmonary vascular remodeling associated with PH in both humans and all animal models, however the mechanisms driving these processes remain poorly understood. Aim 3 will investigate the mechanisms by which complement activated platelets support the recruitment and activation of blood borne monocytes to the pulmonary vasculature promoting pulmonary vascular remodeling and pulmonary hypertension. The overall objective of these studies is to determine whether complement activated platelets drive endothelial and monocyte activation promoting pulmonary vascular remodeling and PH, setting the stage for future studies targeting the platelet immune response with novel currently available therapies.